1. Field of the Invention
The present invention relates to a technique with respect to a ceramic electrostatic chuck for retaining a wafer which is used in plasma CVD, etcher, or the like in a semiconductor manufacturing apparatus, and particularly relates to an electrostatic chuck module in which water is used as a coolant.
2. Description of Background Art
In a semiconductor manufacturing apparatus for plasma CVD, etcher, or the like, high frequency is applied in a state where gas is flowed in a vacuum, so that reaction proceeds. In this instance, since a wafer attracted by an electrostatic chuck is subjected to heat, uniform processing cannot be conducted due to temperature variation of the wafer unless the wafer is cooled. Thus, the electrostatic chuck and a cooling plate have been bonded, and thereby cooling is conducted so as to keep the temperature of the wafer constant.
As the cooling plate for the electrostatic chuck module, aluminum has been used conventionally. In this case, Fluorinert (trademark name) or the like is used as a coolant. However, since Fluorinert has drawbacks of high cost and bad cooling efficiency, an attempt has been made to use water instead of Fluorinert. However, since aluminum has inferior corrosion resistance with respect to water, there is a possibility that a water passage will be clogged or a hole will be opened in the cooling plate due to corrosion. In order to solve these problems, Cu (copper, bronze, etc.) is considered preferable as the material of the cooling plate.
The material of the cooling plate requires high heat conductivity. A composite material including Cu is most suitable to meet this requirement. However, when a leakage test was conducted on a cooling plate which was made of Cu—W, Cu—W—Ni, Cu—Mo, and Cu—Mo—Ni, respectively, a penetration leak occurred in the material itself.
Although an electrostatic chuck has a main feature in that it can be used in a vacuum, it becomes impossible to use an electrostatic chuck in a vacuum in a case where a penetration leak occurs.
Also, since Cu has a different thermal expansion coefficient from ceramic, the surface flatness of the electrostatic chuck is unpreferably varied by temperature variation of the electrostatic chuck due to In (indium) bonding used as a method for bonding metal and ceramic. Incidentally, even in a case of using aluminum, the flatness is varied in the same way. This is because the thermal expansion coefficient is significantly different between alumina ceramic of the electrostatic chuck and metal such as aluminum or copper, and thereby In undergoes plastic deformation by temperature variation.
In particular, when the size of a wafer is as large as 12 inches, the problem of the flatness variation becomes more serious.
In addition, as for the Cu-based composite material, measures need to be taken to prevent corrosion with respect to water including galvanic corrosion which is caused by applying voltage.